In general, in a wireless communication system such as Wideband Code Division Multiple Access (WCDMA) (registered trademark) by Third Generation Partnership Project (3GPP), Long Term Evolution (LTE), LTE-Advanced (LTE-A), and wireless LAN by The Institute of Electrical and Electronics engineers (IEEE), Worldwide Interoperability for Microwave Access (WiMAX), a base station (a cell, a transmission station, a transmission device, and an eNodeB), and a terminal (a mobile terminal, a reception station, a mobile station, a reception device, and User Equipment (UE)) each has a plurality of transmitting and receiving antennas, and spatially multiplexes data signals by using a multi input multi output (MIMO) technology so as to realize high-speed data communication.
In such a wireless communication system, in a case of transmitting downlink data (a transport block for a downlink shared channel (DL-SCH)) to a terminal, the base station multiplexes and transmits demodulation reference signals (DMRS) which are known to the base station and the terminal. Here, the demodulation reference signal is also referred to as a user equipment-specific reference signal (a UE-specific RS, or a terminal-specific RS). Here, the demodulation reference signals are simply denoted as reference signals.
For example, before a precoding process is applied, the reference signal is multiplexed with the downlink data. Therefore, the terminal can measure an equalization channel including the applied precoding process and the propagation path state, by using the reference signal. In other words, the terminal can demodulate the downlink data, even if it is notified of the precoding process that is applied by the base station.
Here, the downlink data is mapped to a physical downlink shared channel (PDSCH). In other words, the reference signal is used to demodulate the PDSCH. Further, for example, the reference signal is transmitted only in resource blocks (referred to as a physical resource block, or a resource) to which the corresponding PDSCH is mapped.
Here, wireless communication system using heterogeneous network (HetNet) deployment which is configured with a macro base station having a wide coverage and a remote radio head (RRH) having coverage narrower than that of the macro base station has been developed. FIG. 13 is a schematic diagram of a wireless communication system using heterogeneous network deployment. As illustrated in FIG. 13, for example, the heterogeneous network is configured with a macro base station 1301, an RRH 1302, and an RRH 1303.
In FIG. 13, the macro base station 1301 has coverage 1305, and the RRH 1302 and the RRH 1303 respectively have coverage 1306 and coverage 1307. Further, the macro base station 1301 is connected with the RRH 1302 through a line 1308, and is connected with the RRH 1303 through a line 1309. Thus, the macro base station 1301 can transmit and receive data signals and control signals (control information) to and from the RRH 1302 and the RRH 1303. Here, for example, a wired line such as an optical fiber and a wireless line using a relay technology are used for the line 1308 and the line 1309. In this case, some or all of the macro base station 1301, the RRH 1302, and RRH 1303 use the same resource, thereby allowing overall frequency utilization efficiency (transmission capacity) in an area of the coverage 1305 to be improved.
Further, when a terminal 1304 is located within the coverage 1306, the terminal 1304 can perform single-cell communication with the RRH 1302. Further, when the terminal 1304 is located near the end of the coverage 1306 (cell edge), measures for co-channel interference from the macro base station 1301 may be required. Here, a method of reducing or suppressing the interference for the terminal 1304 located in the cell edge by performing inter-base station cooperative communication in which adjacent base stations are cooperative with each other has been studied, as multi-cell communication between the macro base station 1301 and the RRH 1302. For example, a cooperative multipoint (COMP) transmission scheme and the like have been considered as measures for reducing or suppressing the interference caused by the inter-base station cooperative communication (NPL 1).